Familiar Letters on Chemistry eBook

LETTER VIII

My dear Sir,

Having attempted in my last letter to explain to you
the simple and admirable office subserved by the oxygen
of the atmosphere in its combination with carbon in
the animal body, I will now proceed to present you
with some remarks upon those materials which sustain
its mechanisms in motion, and keep up their various
functions,—­namely, the Aliments.

If the increase in mass in an animal body, the development
and reproduction of its organs depend upon the blood,
then those substances only which are capable of being
converted into blood can be properly regarded as nourishment.
In order then to ascertain what parts of our food
are nutritious, we must compare the composition of
the blood with the composition of the various articles
taken as food.

Two substances require especial consideration as the
chief ingredients of the blood; one of these separates
immediately from the blood when it is withdrawn from
the circulation.

It is well known that in this case blood coagulates,
and separates into a yellowish liquid, the serum of
the blood, and a gelatinous mass, which adheres to
a rod or stick in soft, elastic fibres, when coagulating
blood is briskly stirred. This is the fibrine
of the blood, which is identical in all its properties
with muscular fibre, when the latter is purified from
all foreign matters.

The second principal ingredient of the blood is contained
in the serum, and gives to this liquid all the properties
of the white of eggs, with which it is indeed identical.
When heated, it coagulates into a white elastic mass,
and the coagulating substance is called albumen.

Fibrine and albumen, the chief ingredients of blood,
contain, in all, seven chemical elements, among which
nitrogen, phosphorus, and sulphur are found.
They contain also the earth of bones. The serum
retains in solution sea salt and other salts of potash
and soda, in which the acids are carbonic, phosphoric,
and sulphuric acids. The globules of the blood
contain fibrine and albumen, along with a red colouring
matter, in which iron is a constant element. Besides
these, the blood contains certain fatty bodies in small
quantity, which differ from ordinary fats in several
of their properties.

Chemical analysis has led to the remarkable result,
that fibrine and albumen contain the same organic
elements united in the same proportion,—­i.e.,
that they are isomeric, their chemical composition—­the
proportion of their ultimate elements—­being
identical. But the difference of their external
properties shows that the particles of which they
are composed are arranged in a different order. (See
Letter V).

This conclusion has lately been beautifully confirmed
by a distinguished physiologist (Denis), who has succeeded
in converting fibrine into albumen, that is, in giving
it the solubility, and coagulability by heat, which
characterise the white of egg.